1. Field of the Invention
The present invention relates to an electrophotographic recording device, such as a printer for forming a multicolor image composed of a combination of different color images, and more particularly, to an electrophotographic recording device for printing a full color image by superimposition transfer of Y, M, C and K color images by means of plural electrostatic recording units. Furthermore, the present invention relates to an electrophotographic recording device having a color matching function for adjusting color slippage and density of a color image into an optimal state.
2. Description of the Related Art
Hitherto, ink-jet type or electrophotographic type printers have been generally spread as color printing devices using electrophotographic recording. In such color printing devices, electrostatic recording units for 4 colors, i.e., black (K), cyan (C), magenta (M), and yellow (Y), are tandem-arranged along a recording paper feed direction. The 4-color electrostatic recording units scan a photosensitive drum optically on the basis of image data to form latent images. The latent images are developed with color toners in a developing unit, and then the latent images are superimposed and transferred, in the order of yellow (Y), magenta (M), cyan (C) and black (K), onto recording paper that is fed at a constant speed. At last, the toners are heated and fixed through a fixing unit. In the printing device having a structure wherein Y, M, C and K electrostatic recording units are tandem-arranged along a recording paper feed direction, in order to improve color printing quality, it is essential to reduce positional slippage of toner images transferred onto recording paper that is being fed by the respective electrostatic recording units to heighten the precision of color matching. Therefore, in the tandem type printing device, toner marks are transferred to its feeding belt and then its sensors read the marks. The writing timing of an LED, a laser diode or the like constituting an exposure device is then changed to perform color matching processing for correcting color slippage and toner densities (U.S. patent application Ser. No. 09/234,455). The correction of the color slippage and the toner densities in such a color printing device, particularly a tandem type color printing device as described above, is performed by either automatic correction function which acts automatically, for example, when the power source of the printing device is turned on or when the cover for exchanging toners is opened or shut, or manual correction function that on the basis of test printing results an operator inputs correction values from an operation panel to make correction without automatic correction function.
FIG. 1 is a flowchart of color matching processing based on automatic correction function in the prior art. Conditions such as correction operation timing are beforehand decided at the stage of forwarding from a factory, and cannot be set by an operator. The automatic correction function usually acts when the power source of a printing device is turned on or when the cover thereof is opened or shut. When the power source of the printing device is turned on, an initial setup is performed in step S1. The initial setup includes the initial setup of its hardware, reading of various set values stored in its nonvolatile memory, and check of abnormality of its circuit (self diagnosis). Subsequently, in step S2, a color matching request is set since color matching processing is necessary at the time of use-start of the device based on turning-on of the power source. When the cover is closed after the cover is opened and the toner unit is exchanged, a color matching request is set in step S2. This is because color matching is necessary against the positional slippage of the LED head of the device. In step S3, initial processing in the printing mechanism unit is performed, in order to perform initial action of printing process and so on to make printing possible. When printing can be performed after the end of the initial processing, the present algorithm goes to a loop of waiting for printing (steps S4-S7). In the top step S4 of this loop of waiting for printing, it is checked whether or not there is a color matching request. If the color matching request is present, color matching processing is performed in step S5. That is, color matching processing is performed only one time in step S5 immediately after the initial processing in step S3. After the color matching processing, it is checked in step S6 whether or not there is a printing request. If the printing request is present, the algorithm goes to step S7 to perform printing. On the other band, in the case that an alarm breaks out during the processing of the printing wait loop S4-S7, non-illustrated alarm processing is performed. In the alarm processing, it is repeatedly checked whether or not causes for the abnormality are removed. When all of the causes for the abnormality are removed, the algorithm is restored from the alarm processing to the printing wait loop. In the case that during the outbreak of the alarm the cover is being opened, the algorithm goes from the cover close stage to the printing wait loop through steps S2 and S3. In this case, the color matching request is set up in step S2. Therefore, the algorithm advances from step 4 to step S5 to carry out color matching processing. However, in the case that the cover is not opened during the outbreak of the alarm, the algorithm returns to the top of the printing wait loop (steps S4-S7) without performing the setup of the color matching request in step S2 and the initial processing in step S3 so as to perform the above-mentioned processing again. In this case, no color matching request is set, so that no color matching in the step S5 is performed. However, in the automatic correction mode, such conventional color matching processing is automatically performed whenever the power source is turned on or whenever the cover is closed. Thus, much time is required for the color matching, so that throughput drops. In the case of printing that does not require precision, for example, test printing, a problem that it takes too much time until the result of the printing is checked arises. On the other hand, all operations are left in the charge of an operator in the manual correction mode. Thus, even if color slippage is generated, no color matching is carried out unless the operator becomes aware of it. Color slippage or change in the density of toner is easily caused by change in the internal temperature in the printing device; however, no color matching is performed unless the operator becomes aware of the change in the internal temperature. Thus, an entire manual correction mode has a problem that color slippage is enlarged as a result of the exchange of the toner or change in the internal temperature. Against the problem of the enlargement of color slippage based on the change in the internal temperature, in a printing device a temperature sensor is set up to perform color matching correction operation on the basis of temperature data from the sensor (JP Publication Number 8286566). However, the relationship between the change in the internal temperature and the amount of color slippage vary dependently on use environment of the printing device, the frequency of printing, and so on. Even if the relationship between the change in the temperature and timing of color matching is qualitatively decided, the color matching operation becomes insufficient depending on the situation so that the quantity of color slippage increases. Contrarily, the frequency of the color matching operation becomes high, causing a problem that throughput drops. Furthermore, the temperature sensor and a control unit for the sensor are mounted so that costs rise. A problem based on malfunction of the temperature sensor also arises.